1. Field of the Invention
The present invention relates to a permanent magnet motor comprising a plurality of permanent magnets provided in a rotor core and a washing machine provided with the permanent magnet motor.
2. Description of the Related Art
Magnetic flux is generated by permanent magnets in a permanent magnet motor of the above-described type. The magnetic flux is interlinked with stator windings. It has been desired that an amount of magnetic flux interlinked with stator windings (an amount of interlinkage magnetic flux) should properly be adjusted according to load driven by the permanent magnet motor. However, permanent magnets provided in permanent magnet motors are generally composed of a single type of permanent magnet. Accordingly, an amount of magnetic flux of the permanent magnets is usually constant. For example, when a permanent magnet motor comprises permanent magnets each with a large coercive force, voltage induced by the permanent magnets during high-speed rotation is rendered excessively high, resulting in possible breakdown of electronic components. On the other hand, when a permanent magnet motor comprises permanent magnets each with a small coercive force, output power during low speed rotation is reduced.
For example, Japanese patent application publication, JP-A-2006-280195 discloses a permanent magnet motor comprising two types of permanent magnets with different coercive forces provided in a rotor core. In the disclosed permanent magnet motor, the permanent magnets with a smaller coercive force are magnetized or demagnetized by an external magnetic field due to armature reaction (a magnetic field generated by electric current flowing into stator winding), whereby an amount of magnetic flux of the permanent magnets is adjusted.
However, both permanent magnets with larger and smaller coercive forces are disposed in each part constituting one magnetic pole inside a rotor core in the permanent magnet motor disclosed by the above-referenced publication. More specifically, the permanent magnet motor of the above-referenced publication is constructed so that one magnetic pole is formed by a plurality of types of permanent magnets. As a result, the number of permanent magnets is increased and a cubic volume of each magnet needs to be rendered smaller, whereupon the structure of the motor is complicated.
In view of the above-described problem, a permanent magnet motor with a simplified structure has recently been proposed. In the proposed motor, two types of permanent magnets having different coercive forces are disposed alternately per pole. In this permanent magnet motor, however, the magnets each having a larger coercive force and the magnets each having a smaller coercive force are in the component ratio of 1:1. Furthermore, a residual flux density of each permanent magnet with the smaller coercive force is smaller than a residual flux density of each permanent magnet with the larger coercive force. Accordingly, a total amount of magnetic flux of the permanent magnet motor is extremely lower than in the case where the rotor of the permanent magnet motor includes only the permanent magnets each having a larger coercive force.
The total amount of magnetic flux of the permanent magnet motor can be increased when the percentage of the permanent magnets with the smaller coercive force in all the permanent magnets is appropriately reduced. However, the amount of magnetic flux is rendered non-uniform when the aforesaid percentage of the permanent magnets each with the smaller coercive force is reduced in a permanent magnet motor wherein one type of permanent magnet is disposed per pole. This increases the torque ripple and cogging torque, resulting in production of noise and/or oscillation or vibration.